This paper presents a model of adult reasoning that is consistent with a set of related psychological theories of learning known collectively as constructivism. Three general factors comprise the model: (1) personal epistemology (defined as beliefs about learning, dispositions toward thinking, and assumptions about knowledge itself); (2) sustained contextual support (defined in terms of practice coping with ill-structured problems); and (3) immediate contextual support (defined in terms of the level of scaffolding provided in a problem solving situation). Although the model has broad applicability for thinking about adult intellectual development, the focus of this paper is on the reflective thinking capabilities of young adults preparing to become teachers. Literature leading to the identification of factors in the model is highlighted, research generated by the model is reviewed, and implications of the model for teacher education are discussed. (Contains 62 references.) (Author)